Silicon carbide (SiC) is one of several advanced ceramic materials which are currently receiving considerable attention as electronic materials, as potential replacements for metals in engines, and for a variety of other applications where high strength, combined with low density and resistance to oxidation, corrosion and thermal degradation at temperatures in excess of 1000.degree. C., are required (K. J. Wynne et al., Ann. Rev. Mater. Sci. 14, 297 (1984)). Unfortunately, these extremely hard, non-melting ceramics are difficult to process by conventional forming, machining, or spinning applications rendering their use for many of these potential applications problematic. In particular, the production of thin films by solution casting, continuous fiber by solution or melt spinning, a SiC matrix composite by liquid phase infiltration, or a monolithic object using a precursor-based binder/sintering aid, all require a source of SiC which is suitable for solution or melt processing and which possesses certain requisite physical and chemical properties which are generally characteristic of polymeric materials.
Polymeric precursors to ceramics such as SiC afford a potential solution to this problem as they would allow the use of conventional processing operations prior to conversion to ceramic (Wynne et al., supra.). A ceramic precursor should be soluble in organic solvents, moldable or spinnable, crosslinkable, and give pure ceramic product in high yield on pyrolysis. Unfortunately, it is difficult to achieve all these goals simultaneously. Currently available SiC precursor systems are lacking in one or more of these areas. Problems have been encountered in efforts to employ the existing polysilane and polycarbosilane precursors to SiC for preparation of SiC fiber and monolithic ceramic objects. All of these precursors have C/Si ratios considerably greater than one, and undergo a complex series of ill-defined thermal decomposition reactions which generally lead to incorporation of excess carbon. The existence of even small amounts of carbon at the grain boundaries within SiC ceramics has been found to have a detrimental effect on the strength of the ceramic, contributing to the relatively low room-temperature tensile strengths typically observed for precursor-derived SiC fibers.
Efforts to develop polymeric precursors to SiC have focused largely on two types of polymers, polysilanes, which have a Si--Si backbone, and polycarbosilanes, in which the polymer backbone is [--Si--C--].sub.n. The polysilanes all suffer from problems due to insolubility, infusibility and/or excess carbon incorporation. Certain of the polycarbosilanes have more suitable physical properties for processing; however, in general, these also contain a higher-than-1:1 C:Si ratio and incorporate excess carbon on pyrolysis.
In the case of the polycarbosilanes, high molecular weight linear polymers of the type [R.sub.2 SiCH.sub.2 ].sub.n, where R is H and/or hydrocarbon groups, have been prepared by ring-opening-polymerization (ROP) reactions starting from cyclic disilacyclobutanes using chloroplatinic acid and related catalyst systems (see W. R. Bamford et al., J. Chem. Soc., C(1966) 1137); however, such linear polycarbosilanes generally exhibit low yields of ceramic product on pyrolysis due to chain "unzipping" reactions (Wynne et al., supra.). For example, studies of high molecular weight [Me.sub.2 SiCH.sub.2 ].sub.n polymers have indicated virtually complete volatilization on pyrolysis under an inert atmosphere to 1000.degree. C. (Wynne et al., supra.). Recent work by the instant inventors on the related [MeHSiCH.sub.2 ].sub.n polymer suggests that the introduction of Si--H groups leads to crosslinking reactions on pyrolysis and, subsequently, higher ceramic yields; however, significant loss of volatile organo-silane byproducts as well as hydrocarbons on pyrolysis was still evident.
Smith (see U.S. Pat. No. 4,631,179) has employed this ROP method to obtain what is claimed to be a linear polymer of the formula [SiH.sub.2 CH.sub.2 ].sub.n. This polymer was reported to exhibit ceramic yields up to 85% on pyrolysis. The starting material for the ROP reaction was the cyclic compound [SiH.sub.2 CH.sub.2 ].sub.2, which is difficult and costly to obtain in pure form by either of the procedures that have been reported (see J. Laane, J. Am. Chem. Soc., 89, 1144 (1967) and R. M. Irwin et al., J. Am. Chem. Soc., 99, 3273 (1976)).